1. Field of the Invention
This invention relates to a disposable syringe comprising a syringe cylinder and a syringe piston, wherein the syringe cylinder is open at one end and at the other end has an end wall, which is closed with the exception of a region adjacent to a needle holder, and the syringe piston comprises a piston skirt, which has a free end that protrudes from the syringe cylinder and at that free end carries a handle, and said piston also comprises a piston head, which is disposed within the syringe cylinder and is provided with an annular seal, which contacts the inside surface of the cylinder and is connected to the piston head by a peripheral annular flange.
2. Description of the Prior Art
Such disposable syringes are generally made of plastic and are intended to ensure an absolutely tight contact between the annular seal and the inside surface of the cylinder during the intake and delivery strokes of the piston and to permit the piston to be actuated gently and with a minimum of force during the actual injection. These two requirements are not satisfactorily met by the existing disposable syringes, in which the annular seals consist of sealing beads integrally formed on the piston head or of flanges provided with two-legged sealing lips (Published German Application 20 25 379; French Patent Specification 15 00 009) so that the seal proper is supported by means which are rather stiff in a radial direction and a sufficiently strong pressure force is exerted between the seal and the inside surface of the cylinder. But that design has a result that the cylinder wall will expand during a storage of the syringe for considerable time so that the sealing action is more or less lost and it is no longer possible to actuate the piston as uniformly as is desired because the strain has resulted in a variation of the diameter of the cylinder. That situation cannot be improved even by a closer fit between the seal and the inside surface of the cylinder because this would result only in a requirement for a stronger force for actuating the piston so that a sensible actuation of the piston would no longer be possible. It has been attempted to eliminate that disadvantage by an incorporation of a solid lubricant in the material of the cylinder. But in that case a long time must be permitted to elapse after the manufacture of the syringe before said solid lubricant emerges from the material for a lubricating action.
In other known syringes the piston head is caplike and fitted on the adapter end of the piston skirt (Swiss Patent Specification 366,126; French Patent Specification 1,228,933) and constitutes a protruding conical sealing lip because its peripheral surface defines a forwardly open annular wedge-shaped gap and said sealing lip is integrally formed with an oppositely directed, second sealing lip. In order to ensure the required tightness in both directions in which the piston is actuated, the sealing lips must contact the inside surface of the cylinder under a sufficiently strong pressure force so that the seal has a high stiffness, with all the disadvantages involved therein. That stiffness is further increased by the fact that the piston head must be sufficiently firmly fitted on the piston skirt.
All said known disposable syringes also involve the risk that a leak may result from a canting of the piston as the syringe is actuated. Such a canting cannot be prevented because there is a radial clearance between the piston skirt and the syringe cylinder and because the syringe is made of flexible material.